I haven't been officially diagnosed with myotonia yet, but what I call "stiffness" I measure according to how much it disables me. If I feel "weird" when moving, or slow, then that's not too bad, whereas if it takes all my effort to move a body part, then that is really bad. So I suppose for me it's entirely subjective. I'm not sure how you could measure it objectively, short of having access to lab equipment or something. Maybe you could figure out how fast you can perform an action?